The invention relates to a multifocal lens, having a reading part in the bottom part and another lens in the top part. This lens can be a hard or soft contact lens as well as an intra ocular lens.
A multifocal lens is known from U.S. Pat. No. 4,854,089. In the general prior art it is known to provide a part with smaller radius of curvature in the centre of the lens. This part with smaller radius of curvature is, as it were, placed on a larger part with a larger radius of curvature. This is a rotationally symmetrical lens where there is no need for stabilizing means. Due to the fact that there is a kink at the position of the Rlecs-Rverte (reading-distance) transition, a great deal of reflection occurs in the direction of the optical axis, which means that the optical characteristics of this lens are inadequate.
Designs are also known from the prior art in which a reading part is moulded or cast into the lens material, or is disposed on the front side in a half moon shape. Such systems can work only if the lens can move fully from distant to close up. The reading part covers about 50% of the pupil if the lens is centred on the cornea. In order to obtain an undistorted image, the lens will have to move downwards until it is at or below the lower eyelid. The lens has to move up again for reading. These systems are characterized in that the optical axes do not lie level, with the result that an image discontinuity always occurs. This causes an unsteady image for the wearer, and leads to complaints of double vision and shadow images.
In another system (tangent streak) the optical axis does remain level. It lies precisely in the centre of the lens. The disadvantage of this lens is that the distance/close-up division is always 50%. The lens is divided exactly into two halves. This optical system is characterized in that the two outer radii for the reading and distance part are monocurved, i.e. they are spherical from the centre to the edge. This produces a very thick lens. This design also means that at the edge quite a large height difference arises between the two curves. Both have a very negative influence on wearing and vision comfort.
On the other hand, the reading part on the lens is designed as a sort of half moon. The characteristic feature of this lens is the enormous image discontinuity which occurs. The reason for this lies in the fact that the lens has to be prismatic, in order to obtain a stable position on the eye, on the one hand, and in order to leave sufficient material to be able to make the reading part, on the other hand, This prismatic effect is present optically in the distance part. However, the reading part is not provided in the second instance with prism effect. The optical centre points do not lie on the optical axis, nor can they be joined tangentially in the centre. A further disadvantage is that the shape of the xe2x80x98half moonxe2x80x99 on the front side, which is the reading part, is not adjustable. This is a resultant. The dimensions of this part are different in the case of each lens power combination, with the result that much too large a reading part and too small a distance zone are produced. The consequence is that this lens has many reflections for the user. The lens requires a great vertical movement in order to function. This causes a high degree of discomfort for the user.
It is therefore the object of the present invention to provide a multifocal lens which has bivisual features, without the presence of a kink or similar transition which causes troublesome reflections in the direction of the optical axis or an image discontinuity, and which lens has such features that no great movement on the cornea is needed for fulfilment of the distance and reading functions.
According to one aspect of the invention, a multifocal lens is provided with a reading portion in the lower lens part and another lens portion in its upper part, wherein the reading portion comprises a recess which is bounded on all sides by the remaining lens part, and in that the outer limit of the reading portion lies on or within an imaginary sphere having its origin and radius of curvature coinciding with the radius Rv of said other lens portion.
The reading part is taken from the remaining part of the lens, i.e. instead of the lens being provided with a thickening for the reading part, it is provided with a thinner part or recess. This means that an extremely gradual transition between reading and distance part can be provided, with the result that irritation, image discontinuity and reflection are avoided. The lens need not move any more than an ordinary monofocal lens, because the distance/close-up ratio in the lens system can be selected in the optimum way. A ratio with 30-40% reading part and 60-70% distance part is a good value. In some cases where lenses are in a low position on the eye, another ratio, for example 50% reading and 50% distance, can give a better solution. The size and position and shape of the zones can be tailored exactly by the invention to the needs of the lens wearer. There is no transition in the centre of the lens because the two curves merge into each other at the position of the transition, i.e. the angle at which the two curves touch each other is the same. The height difference at a occurring further position in the surface is reduced sinusoidally without an optically appreciable effect, or is reduced in such a way that optically an effectively working intermediate zone is produced. An optically working transition zone is then produced between sectors with multifocal effect. This means that a lens can be made extremely thin, so that the oxygen permeability of the lens is also optimal. At the outer periphery of the lens, it will have the dimensions of the remaining part of the lens surface, i.e. there will be a transition zone between the outer periphery of the lens and the reading part. Irritation is avoided in this way.
According to a further aspect of the invention, provision is made for a multifocal lens which is provided with a reading part in the usual way at the bottom side. According to an further aspect of the invention, at least one additional reading part, which is disposed opposite (relative to the axis of the lens) the first reading part is provided. This means that the stabilization of the lens is less critical, or may even be superfluous. This additional reading part can have the same power as or a different power from the first reading part.
Location of the lens is of course of importance. In a contact lens to that end stabilisations means are provided. According to a further aspect of the invention, the lens does comprise an intra ocular lens which is positioned through locating means in the lens cavity of the eye.
Such a lens is preferably provided with the position-stabilizing means to be described below, but it must be understood that it is also possible to work with other position-stabilizing means known in the prior art. It has been found that the human eye becomes accustomed to the presence of such a second reading part, so that the position of the lens on the eye becomes less critical.
Both the reading part and the distance part are preferably symmetrical relative to the vertical axis of the lens in the use position Moreover, a transition zone, comprising 10-30% of the surface area of the distance part, is preferably present between the reading part and the distance part.
The position-stabilizing means described above can comprise any designs known in the art. With bifocal lenses and other lenses which are not rotationally symmetrical, it is important in some way to fix the position of such a lens on the cornea. In the abovementioned European patent application it is proposed that elevations should be disposed on the horizontal axis of symmetry. This achieves stabilization each time that the eyelids are closed. The height of such elevations relative to the remaining part of the lens lies between 0.1 and 0.2 mm.
The condition for proper functioning of such a design is that the user must close his eyelids sufficiently often.
In this case, stabilization of the lens occurs only during the closing movement of the eyes, and no correction occurs when the eyes are open. If the time between two blinks is relatively long, such stabilization does not work properly.
Reduction of the height has proved not to be possible, because in that case it cannot be ensured adequately that the lens will always be in the correct position. That is why such lenses have not become generally accepted.
Use of ridges is not possible in the case of the relatively smaller, so-called hard contact lens, because such a lens rests freely on the lower eyelid.
In order to obtain sufficient mass, the thickenings are designed with considerable height and width. This leads to irritation of the lower eyelid. In order to avoid this problem, it is proposed that other types of thickenings, such as prisms, should be provided in the contact lens. The prism results not only in a considerable increase in thickness in the bottom pat of the lens, but also leads to a so-called optical image height discontinuity, where the user receives an optical height difference relative to the other eye. This difference can be as much as 2 cm per metre, and is particularly troublesome. Owing to the fact that the thickness of the lens increases, the oxygen permeability and wearing comfort will be reduced.
Improved stabilization of the contact lens is obtained according. to the invention in that:
said elevations are provided above the horizontal axis of the lens,
said elevations have a height of between 0.05 and 0.1 mm, and
further stabilizing means are present, disposed in the bottom part of the lens and comprising a part with increased weight.
Both dynamic and static stabilization are obtained with the currently proposed design The dynamic stabilization is achieved by the elevations, However, in contrast to the prior art, these elevations are relatively small in size because, if the position for these elevations is chosen too high up, the lens will no longer move towards a central position after the blink, due to the fact that the upper eyelid will hold the lens. The relatively low elevations mean that no irritation will be experienced. The weight-increasing stabilizing means serve for static stabilization. The latter can likewise be made more limited, so that, the abovementioned disadvantages connected with them do not occur.
Positioning the ridges above the horizontal axis ensures that a secure position orientation is obtained at all times, owing to the fact that said ridges always rest lightly against the upper eyelid or sit slightly below it. This also means that when the eye is open the lens in combination with the heavier part in the bottom thereof will remain in position. It has also been found that the upper eyelids make a greater sweep than the lower eyelids, and consequently largely cover the lens during a blink. In the case of hard lenses in particular, i.e. lenses with relatively small diameter, elevations above the horizontal axis are advantageous, because said elevations are not or are barely reached by the lower eyelid.
The static stabilizing means described above can be all types known in the prior art, such as ridges and prisms. Furthermore, they can be in the form of the peripheral edge thickening. Combinations of these variants are possible.
Since a peripheral edge thickening can have a lower weight on account of the presence of the dynamic stabilizing means, provision can be made for a transition zone between the peripheral edge thickening and the remaining part of the lens. This means that irritation is not longer encountered as a result of the abrupt transition. However, in the construction according to this German utility model such a transition is unavoidable because of the amount of mass which has to be put into the peripheral edge thickening.
This transition zone can be a sinusoidal shape, i.e. the transitions are extremely smooth.
The height of the peripheral edge thickening preferably lies between 0.1 and 0.4 mm.
The transition zone of the peripheral edge thickening described above can be relatively long, and can lie between +70-0xc2x0 and 270xc2x0, while the vertical axis is 90-270xc2x0. Of course, the mirror-inverted range is possible, which gives a range of 110-180-270xc2x0. These transition zones do not have to be mirror-inverted.
It has been found that a lens designed in this way has good position stabilization and is particularly comfortable to wear.
It should be understood that these stabilizing means can be achieved independently of the multifocal contact lens described above with special transition between the distance part and the reading part.
The invention also relates to a method for producing a contact lens, comprising production by tuning, during which the contact lens blank is placed on a machining holder and subjected to the influence of one or more material-removing devices,
In the prior art elevations and other surface irregularities were produced by means of combined turning and milling operations.
This treatment made the production of such lenses extremely expensive and placed limits on their design. Spin cast and mould cast were mentioned as alternatives. Spin casting is a method using a rotating mould, which is filled with liquid monomer material and the liquid monomer is hardened by exposure to ultraviolet light. In the mould casting process a forming die is produced by injection moulding, and the forming die is used as the mould for liquid lens material, which subsequently hardens. There is no point in such methods of production unless there are large production runs, and they are technically difficult to achieve, due to the fact that the moulds are very difficult or impossible to produce.
The object is to improve the method described above in such a way that it becomes possible in a relatively simple manner to produce small production runs or forming dies for lenses, a few contact lenses or smaller production runs.
This object is achieved with a method described above in that that during the turning the rotating lens or forming die is subjected to a to and fro movement relative to said material-removing device in the direction of the axis of rotation, in order to form at least one of the elevations, peripheral edge thickening or reading lens.
In this process the lens or forming die preferably moves to and fro with a certain frequency. and stoke, such as a frequency of maximum 200 Hz and a stroke of approximately 0.3 mm. Owing to the fact that the lens moves, and not the tool, different tools can be used for different operations.
The material-removing device can comprise a conventional tool, but it is also possible to remove material from the lens blank by means of laser light. The to and fro movement of the lens or forming die described above relative to the material movement direction can mean that the lens moves to and fro relative to the environment and a tool does not make such a movement, but the reversexe2x80x94where the tool moves to and fro relative to the environmentxe2x80x94is also possible. For the production of forming dies the cast mould or spin cast process can subsequently be used.